1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and particularly to a liquid crystal display based on an MVA (Multi-domain Vertical Alignment) mode of multi-division alignment in which alignment states of liquid crystal molecules having a negative dielectric anisotropy are made different from each other.
2. Description of the Related Art
An LCD is regarded as the most promising substitute for a CRT among various flat panel displays. It is expected that the LCD has an extended market by being applied to not only a display monitor of a PC (Personal Computer), a word processor, or an OA equipment, but also a display portion of a consumer (household electric) appliance such as a large screen television or a portable small-sized television.
A display operation mode of the LCD, which is most frequently used at present, is a normally-white mode using a TN (Twisted Nematic) liquid crystal. This LCD includes electrodes respectively formed on opposite surfaces of two glass substrates arranged opposite to each other, and horizontal alignment films formed on both the electrodes. The two horizontal alignment films are subjected to an alignment processing by rubbing or the like in the directions perpendicular to each other. Besides, polarizing plates having polarization axes adjusted to be parallel to the rubbing directions of the alignment films of the inner surfaces of the substrates are respectively arranged at the respective outer surfaces of the substrates.
When a nematic liquid crystal having a positive dielectric anisotropy is sealed between the substrates, liquid crystal molecules in contact with the alignment film are aligned in the rubbing direction. That is, the alignment directions of the liquid crystal molecules being in contact with the two alignment films cross at right angles. At the same time as that, the liquid crystal molecules between both the substrates are lined up in the direction vertical to the substrate surface while the alignment direction is successively rotated in a plane parallel to the substrate surface, and the liquid crystal is twisted with a twist angle of 90° between the substrates and is lined up.
If light is made incident on one of the substrate surfaces of the TN type LCD of the above construction, when linearly polarized light having passed through the polarizing plate at the side of the one substrate passes through the liquid crystal layer, the polarization orientation rotates in an arc of 90° along the twist of the liquid crystal molecules, and the light passes through the polarizing plate at the side of the other substrate having the polarization axis orthogonal to the polarizing plate at the side of the one substrate. By this, a bright state display can be obtained at the time of no voltage application (normally-white mode).
When a voltage is applied between the opposite electrodes, since major axes of the nematic liquid crystal molecules having the positive dielectric anisotropy are aligned perpendicularly to the substrate surface, the twist disappears. The liquid crystal molecules do not have birefringence (refractive index anisotropy) relative to the linearly polarized light incident on the liquid crystal layer of this state. Accordingly, since the incident light does not change its polarization direction, it can not pass through the other polarizing plate. By this, a dark state display is obtained at the time of predetermined maximum voltage application. When the state is again made the no voltage application state, it is possible to return the display to the bright state display by alignment regulating force. Besides, a gradation display becomes possible by changing the applied voltage to control the tilt of the liquid crystal molecule and to change the intensity of transmitted light from the other polarizing plate.
An active matrix TN type TFT-LCD in which a TFT (Thin Film Transistor) is provided in each pixel as a switching element for controlling an applied voltage between opposite electrodes for each pixel is widely used for a PC display monitor, a portable television or the like since it is thin and lightweight, and a large screen and high quality can be obtained. A manufacturing technique of the TN type TFT-LCD is remarkably advanced in recent years, and the contrast, color reproduction property and the like when looking at the screen front ways are superior to a CRT. However, the TN type TFT-LCD has a fatal defect that a viewing angle is narrow. Especially, the viewing angle in the vertical direction is narrow in panel observation directions. The luminance of a dark state increases in one direction and an image becomes whitish, and a dark display is obtained as a whole in the other direction, and a luminance inversion phenomenon of an image occurs at a halftone. This is the biggest defect of the TN type LCD.
As an LCD which has solved the problem of the viewing angle characteristics of the TN type LCD, there is an MVA-LCD disclosed in Japanese Patent No. 2947350. An example of the construction of the MVA-LCD will be described. First, electrodes are respectively formed at the sides of opposite surfaces of two substrates having a predetermined gap and arranged opposite to each other. Vertical alignment films are formed on both the electrodes, and a liquid crystal having a negative dielectric anisotropy is sealed between the two vertical alignment films. A plurality of linear projections made of insulators are periodically formed between the electrodes and the vertical alignment films of both the substrates. The linear projections opposite to each other between the two substrates are arranged while they are shifted by a half pitch from each other when viewed from the substrate surface. The linear projections are used for alignment control to divide the liquid crystal in a pixel region into plural alignment orientations. Incidentally, even if slit portions are provided on the electrodes instead of the linear projections, the alignment division can be controlled.
Two polarizing plates having polarization axes orthogonal to each other are provided at the outer surfaces of the two substrates. The attachment directions of the polarizing plates are adjusted so that the orientations of the major axes of the liquid crystal molecules tilted on the substrate display surfaces at the time of voltage application become approximately 45° with respect to the polarization axes of the polarizing plates when viewed from the substrate surface.
When a nematic liquid crystal having a negative dielectric anisotropy is sealed between the substrates, the major axes of the liquid crystal molecules are aligned in the vertical direction with respect to the film surface of the vertical orientation film. Thus, the liquid crystal molecules on the substrate surface are aligned vertically to the substrate surface, and the liquid crystal molecules on the tilted surfaces of the linear projections are aligned obliquely to the substrate surface.
When light is made incident on one of the substrate surfaces in the state where a voltage is not applied between both the electrodes of the MVA-LCD of the above construction, the linearly polarized light having passed through the one polarizing plate and incident on the liquid crystal layer travels in the directions of the major axes of the vertically aligned liquid crystal molecules. Since birefringence does not occur in the directions of the major axes of the liquid crystal molecules, the incident light travels without changing the polarization orientation, and is absorbed by the other polarizing plate having the polarization axis orthogonal to that of the one polarizing plate. By this, a dark state display is obtained at the time of no voltage application (normally-black mode).
When a voltage is applied between the opposite electrodes, the major axes of the liquid crystal molecules are aligned parallel to the substrate surface while the alignment orientations of the liquid crystal molecules on the substrate surface are regulated in accordance with the alignment orientations of the liquid crystal molecules tilted by the linear projections in advance.
The liquid crystal molecule has birefringence relative to the linearly polarized light incident on the liquid crystal layer of this state, and the polarization state of the incident light is changed according to the tilt of the liquid crystal molecule. At the time of predetermined maximum voltage application, since the light passing through the liquid crystal layer becomes, for example, linearly polarized light in which the polarization orientation rotates in an arc of 90°, it passes through the other polarizing plate and a bright state display is obtained. When the state of no voltage application is again produced, the display can be returned to the dark state display by the alignment regulating force. Besides, a gradation display becomes possible by changing the applied voltage to control the tilt of the liquid crystal molecule and to change the intensity of transmitted light from the other polarizing plate.
According to the active matrix MVA system TFT-LCD in which a TFT is formed in each pixel, since the alignment orientation of the liquid crystal in the pixel can be divided into plural ones, as compared with the TN type TFT-LCD, an extremely wide viewing angle and high contrast can be realized. Besides, since a rubbing processing is not required, the manufacturing process becomes easy and the manufacturing yield can be improved.
However, the conventional MVA system TFT-LCD can be improved in the response time of a display. That is, a high speed response can be made in the case where black is again displayed after a black display was changed to a white display. However, a response time when a halftone is displayed from another halftone is rather inferior to the TN type TFT-LCD.
Besides, also with respect to the transmittance of light, although the conventional MVA system TFT-LCD is substantially twice as excellent as a wide viewing angle LCD of an IPS (In-plane Switching) system of a horizontal electric field system, it is inferior to the TN type TFT-LCD.
As stated above, although the MVA system TFT-LCD has solved the problem of the conventional LCD with respect to the viewing angle, contrast, and response time of black-white-black display, it does not exceed the conventional TN type LCD with respect to the response time at a halftone display and the transmittance.
Here, the reason why the halftone response of the conventional MVA-LCD is slower than the conventional TN type LCD will be described with reference to FIGS. 73A to 74C. FIGS. 73A to 73C show a schematic construction of a section obtained by cutting an MVA-LCD panel in the direction vertical to a substrate surface. FIG. 73A shows an alignment state of liquid crystal at the time of no voltage application, and FIG. 73B shows an alignment state of the liquid crystal at the time of voltage application. FIG. 73C is a conceptual view showing an alignment control state. FIGS. 74A to 74C show a schematic construction of a section obtained by cutting a TN type LCD panel in the direction vertical to a substrate surface. FIG. 74A shows an alignment state of liquid crystal at the time of no voltage application, and FIG. 74B shows an alignment state of the liquid crystal at the time of voltage application. FIG. 74C is a conceptual view showing an alignment control state.
First, a TN type LCD 100 will be described with reference to FIGS. 74A to 74C. As shown in FIG. 74A, at the time of no voltage application, a liquid crystal 102 of the TN type LCD 100 is twisted with a twist angle of 90° and is aligned between an electrode 108 at the side of an upper substrate 104 and an electrode 110 (either alignment film is not shown) at the side of a lower substrate 106 arranged opposite to each other. When a voltage is applied between the electrodes 108 and 110, as shown in FIG. 74B, liquid crystal molecules rise almost vertically to the surfaces of the substrates 104 and 106 and the twist disappears. If the voltage application is removed, the liquid crystal molecules rotate in the direction substantially parallel to the original surfaces of the substrates 104 and 106 and return to the twist alignment. As stated above, in the case of the TN type LCD 100, as shown by an oblique line portion 112 of FIG. 74C, we can consider that not only the liquid crystal molecules in the vicinity of the interfaces of the not-shown alignment films on the electrodes 108 and 110 are alignment-controlled by the regulating forces of the alignment films, but also the liquid crystal molecules in the center region of the liquid crystal layer 102 are also alignment-controlled to a certain degree by a twist alignment due to addition of a chiral agent or the like.
On the other hand, as shown in FIG. 73A, at the time of no voltage application, in a liquid crystal 124 of an MVA-LCD 114, liquid crystal molecules other than those in the vicinity of linear projections 126, 128, and 130 are almost vertically aligned between an electrode 120 at the side of an upper substrate 116 and an electrode 122 (either alignment film is not shown) at the side of a lower substrate 118 arranged opposite to each other. The liquid crystal molecules in the vicinity of the linear projections 126 to 130 are aligned almost vertically to the surfaces of the not-shown alignment films on the oblique surfaces of the projections and are tilted with respect to the substrate surfaces. When a voltage is applied between the electrodes 120 and 122, as shown in FIG. 73B, the tilt of the liquid crystal is successively propagated in the tilt directions of the liquid crystal molecules in the vicinity of the linear projections 126 to 130 for alignment regulation. Thus, a time lag occurs until the liquid crystal in a portion between a linear projection and an adjacent linear projection, that is, in the center of the gap portion finishes tilting. Especially, in the gradation change from black to a dark halftone, the change amount of applied voltage is small and the change of the intensity of an electric field in the liquid crystal is small, so that the propagation speed of the tilt of the liquid crystal molecule is lowered.
Falling directions of the liquid crystal molecules existing in the space portions of the linear projections 126 to 130 are not determined if the tilt direction is not propagated from the linear projections 126 to 130. That is, as shown by oblique line portions 132 of FIG. 73C, the alignment of the liquid crystal in the MVA-LCD is regulated by only the distortion of an electric field in the vicinity of the interfaces of the alignment films to which the regulating forces of the alignment films on the substrate surfaces reach, and at the alignment films on the linear projections 126 to 130 and their vicinity, and the liquid crystal alignment of the other region is only indirectly controlled.
Even in the conventional MVA construction, if the space distance (pitch) of the linear projections of the upper and lower substrates is made short, the response time can be made short. However, as described above, in a general MVA-LCD, since the tilt orientation of liquid crystal is determined by the projection oblique surface of an insulator, the tilt portion must have a certain degree of width, length and height. Thus, the pitch of the upper and lower projections cannot be made very short.
FIG. 75 shows an alignment state of liquid crystal molecules at the time of voltage application when the MVA-LCD shown in FIGS. 73A to 73C is viewed from the side of the lower substrate 118. Among the three linear projections 126, 128 and 130 extending horizontally in the drawing, the upper and lower two projections 126 and 128 are formed on the lower substrate 118, and the center one projection 130 is formed on the upper substrate 116.
The liquid crystal molecules, which are aligned substantially vertically to the substrates 116 and 118 at the time of no voltage application, are alignment-divided, as shown in FIG. 75, at the time of voltage application into an alignment region A in which they are aligned in the direction (upward direction of the paper plane) toward the linear projection 128 at the side of the lower substrate 118 from the linear projection 130 at the side of the upper substrate 116, and an alignment region B in which they are aligned in the direction (downward direction of the paper plane) toward the linear projection 126 at the side of the lower substrate 118 from the linear projection 130.
That is, at the time of voltage application, the liquid crystal molecules over the adjacent alignment regions A and B at both sides of the linear projection 130 are alignment-divided so that the orientation of the major axis of the liquid crystal of the alignment region A becomes substantially +90° with respect to the extending direction of the linear projection 130, and the orientation of the major axis of the liquid crystal of the alignment region B becomes substantially −90° with respect to the extending direction of the linear projection 130. On the other hand, at the time of voltage application, the liquid crystal molecules in the vicinity of tops of the linear projections 126 to 130 are tilted in the directions in which the respective projections extend, and they are aligned so that the alignment orientation becomes substantially 0° or 180° (parallel) with respect to the extending directions of the respective linear projections 126, 128 and 130.
As stated above, at the time of voltage application, with respect to the alignment orientations (substantially 0° or 180° with respect to the extending directions of the linear projections 126 to 130) of the liquid crystal molecules in the vicinity of the tops of the linear projections 126, 128 and 130, the alignment orientations of the liquid crystal molecules of the display region on the substrates 116 and 118 come to have a state in which they rotate in an arc of 90°. Thus, as shown in FIG. 75, the liquid crystal molecules aligned in the orientation of 45° with respect to the extending directions of the respective linear projections 126, 128 and 130 are arranged at both sides of the tilt surfaces of the linear projections 126 to 130. However, polarization axes P and A of polarizing plates indicated by orthogonal arrows in the drawing are arranged to be tilted with an angle of 45° with respect to the alignment orientations of the liquid crystal molecules of the display regions A and B on the substrates 116 and 118.
Accordingly, since the alignment orientations of the liquid crystal molecules aligned in the orientation of 45° with respect to the respective linear projections 126, 128 and 130 become parallel to and orthogonal to the polarization orientations of the polarization axes P and A of the polarizing plates, as shown by broken lines in the drawing, two dark lines (discrimination lines) 140 and 142 are generated at both sides of the tilt surfaces of the linear projections 126 to 130. Incidentally, the two dark lines 140 and 142 are formed for every interval between a first singular point (indicated by (+1) in the drawing) and a second singular point (indicated by (−1) in the drawing) of alignment vector fields formed on the linear projections 126 to 130. At the first singular point (+1), the orientations of the major axes of the liquid crystal molecules are directed toward substantially the same point, and at the second singularity point (−1), part of the liquid crystal molecules are directed in different directions.
In the conventional MVA-LCD like this, if an attempt to shorten the response time of a halftone is made by shortening the pitch of the upper and lower projections to increase the formation density, not only the occupied area of the projections in the pixel region is increased, but also the formation density of the two dark lines 140 and 142 formed at both sides of the projection is also increased, and a drop in transmittance becomes so large that it can not be neglected. Accordingly, there arises a problem that if the formation density of the linear projections is made high in order to improve the response characteristics of the liquid crystal, the transmittance is lowered. As stated above, the conventional MVA-LCD construction has a problem that the improvement of the response characteristics of the liquid crystal and the improvement of the transmittance have a trade-off relation.